In recent years, environmental problems such as global warming due to release of carbon dioxide gas accompanying use of fossil fuels, radioactive contamination caused by accidents in atomic power plants and nuclear waste are serious, and the spotlight centers on terrestrial environment and energy. In this situation, a solar cell utilizing sunlight as an inexhaustible and clean energy source and the like is in practical use throughout the world.
In a solar battery, the output electric power fluctuates widely depending on the intensity of the sunlight striking the photosensitive surface of the cell. Consequently, when tying a power generation means having such sharp output fluctuation to a power system (hereinafter also called simply a “system”), in order to stabilize the frequency and voltage of the system it is necessary to stabilize the output using a power storage apparatus.
Through this output stabilization, the degradation of the quality of the frequency and voltage of the system due to the following two factors is redressed. One factor is rapid output fluctuation on the power generation side, and the other factor is fluctuation in the load connected to the system.
Numerous control methods and systems for this type of output stabilization have been proposed. For example, Patent Publication No. 2-61227 describes, when tying a solar battery to a system, temporarily storing the output of the solar battery in a battery or the like so that the sharp fluctuations in the output of the solar battery do not affect the system, detecting the solar battery output or sunlight, and controlling the output of a grid tied inverter so that this becomes a smooth value.
In addition, Japanese Laid-Open Patent Publication No. 63-186538 describes a method of attaching a system stabilization apparatus consisting of a battery and a two-way converter to the power system and, when a load fluctuation occurs, adjusting the system stabilization apparatus output.
In other words, where system output fluctuations occurring when tying a solar power generation system affect the load, this method supplies power from the system stabilizing apparatus when the power is insufficient and carries out load leveling that stores power in the power storage apparatus when the power is excessive.
By contrast, ordinarily, a plurality of power generators (hereinafter called “system power generating means”) are connected to the power system side. The operation (that is, the starting and stopping) of these power generators is decided by a supply and demand plan determined in advance. The supply and demand plan is usually drawn up taking past demand fluctuations into account, and the power generators are started and stopped in accordance with the plan.
However, when connecting (that is, tying to a grid) an unstable power generating means such as a solar battery to a system in which the system power generating means are controlled according to a supply and demand plan based simply on actual past loads as described above, because the electric power generated by an unstable power generating means such as a solar battery cannot be relied upon, that electric power must be estimated as much lower than at ordinary operating times. As a result, when a power generating means with the unstable output of a solar battery or the like is connected, in terms of the operation of the power generators, the amount of waste is very great.
In order to eliminate this waste, the output of a power generating means with the unstable output of a solar battery or the like could be stabilized using a power storage apparatus like that described in Japanese Laid-Open Patent Publication No. 63-186538 described above.
However, the method described in Japanese Laid-Open Patent Publication No. 63-186538 assumes that the power generators provided on the system basically all generate power in accordance with a fixed supply and demand plan. Consequently, for example, in a case in which the output of the solar battery and the like is greatly affected over a wide area by a hurricane or other sudden localized weather disturbance, for safety's sake the capacity of the power storage apparatus must be made sufficiently large, thus raising the cost of a power generation system equipped with an unstable power generating means such as a solar battery or the like.